1. Field of the Invention
The present invention relates to an apparatus for extracting a composite video signal of a selected channel from a time divisional multiplexed video signal, and more particularly, to an apparatus for extracting a composite video signal of a selected channel from a time divisional multiplexed video signal into which a channel identification code is inserted for identifying composite video signals of different channels.
2. Description of Related Art
Generally, a frame switcher is used for recording video signals picked up by a plurality of video cameras on a video tape through a single video cassette recorder (VCR). The frame switcher receives video signals picked up by eight cameras through eight channels and sequentially time-division-multiplexes the received video signals frame by frame, to then supply the resultant signals to a single VCR. Such a frame switcher inserts a channel identification code indicative of each channel for identifying eight channels, in a vertical blanking period of a composite video signal. The channel identification code is repeatedly loaded between 12H and 23H in the vertical blanking period. Here, "H" represents a horizontal scanning interval. A time divisional multiplexed video signal output from the frame switcher is recorded on a video tape by a VCR. An operation of a conventional frame switcher will be described below with reference to FIG. 1, when the video signal which is recorded on the video tape in the above manner is to be played back.
In the conventional frame switcher shown in FIG. 1, a channel code detector 10 detects a plurality of channel identification codes contained in a video signal played back from a video tape. Such a channel identification code is composed of three bits when eight channels are to be identified. A sync separator 20 separates a vertical sync signal VD and a horizontal sync signal HD from a playback video signal. Horizontal sync signal HD is applied to a counter 30, and vertical sync signal VD is applied to a shift register 40. Counter 30 counts horizontal sync signal HD input from sync separator 20, and supplies a channel identification code detected by channel code detector 10 to shift register 40 when the counted value becomes a value corresponding to sixteen horizontal scanning intervals. Shift register 40 is cleared by vertical sync signal VD and stores the detected channel identification code. A microcomputer 50 generates a channel identification code corresponding to a user key input signal. A comparator 60 compares the detected channel identification code stored in shift register 40 with the channel identification code output from microcomputer 50. If the compared two data values are same, comparator 60 outputs a high-level pulse signal to a video signal processor 70. Video signal processor 70 processes a playback video signal which is designated by the high-level pulse signal output from comparator 60 among the playback video signals, that is, a playback video signal of a frame having a channel identification code of a user selected channel among the playback video signals, and outputs the processed playback video signal to a monitor (not shown).
When a VCR reads out a channel identification code which has been deformed due to tracking noise, the frame switcher is also to generate a high-level pulse signal according to a deformed channel identification code. Thus, the composite video signal output from an apparatus which has been designed so that the composite video signal of the selected channel from the playback video signal based on a single channel identification code detected by the FIG. 1 apparatus is extracted, has a problem that an image of one frame is erased or an image of a non-selected channel is displayed.